Effect of Ethylene Glycol, Ethanol and Methanol On PVCap-Induced Hydrate Crystal Growth Inhibition in Methane Systems

Methanol (MeOH), mono-ethylene-glycol (MEG) and increasingly ethanol (EtOH) are the most widely used thermodynamic inhibitors (THIs) for hydrate inhibition in hydrocarbon production operations. However, the use of thermodynamic inhibitors can have the disadvantages in that large quantities of inhibitor may be required, meaning large storage, regeneration and injection facilities which increase CAPEX/OPEX, in addition to environmental concerns. As a result, low dosage Kinetic Hydrate Inhibitors (KHIs) are seeing increasing use as a potential alternative for hydrate prevention, either partly or wholly replacing THIs. Combining KHIs with THIs offers a potential means to increase the subcooling to which KHIs can be used (THI acts as a ‘top-up’ inhibitor), while KHIs can be potentially used to reduce THI volumes. If the benefits of such combinations are to be realised, a better understanding of combined performance is required. Here, we report the results of experimental studies of combined KHI (PVCap, poly-n-vinylcaprolactam) – THI methane hydrate inhibition performance for MEG, MeOH and EtOH measured using a Crystal Growth Inhibition (CGI) method previously developed in this laboratory. Results show that neither MeOH nor EtOH act as full ‘top-up’ thermodynamic inhibitors for PVCap; KHI-induced CGI regions are consistently reduced to lower subcoolings as THI concentration is increased in both cases, although the negative effect is more pronounced with EtOH. In contrast, MEG was found to consistently act as a full ‘top-up’ thermodynamic inhibitor to PVCap; CGI regions being larger or equal to those for PVCap alone, and being present on top of the thermodynamic inhibition offered by MEG up to concentrations of 50 mass%. Furthermore, MEG has an increasingly synergistic effect on PVCap as the concentration is increased, reducing hydrate growth rates in CGI regions where growth does occur.